I experienced one of the creepiest bug I had in a survival map recently in the last snapshot. I tried to search it on the bug tracker but I didn't found it. Sorry if it's a duplicated report.

I was in a Mineshaft and because of my dumbness gravel fallen and killed me. I tried to recover my stuff, which had disappear for no reason. When I came back I was in a 1 block hole and just one block of gravel have fallen on me. Then I sneaked and I suddenly was under the bedrock, like I am able to pass trough all block like a falling block entity trough the air.

One thing I noticed is that the animation of sneaking is broken when the falling block entity has changed in a block before we sneak again to pass through the blocks

Tried in creative, survival and I think it's the same for adventure mode 
Here's a video to show exactly how to reproduce it.